Compressed gas can be used as a fuel to provide benefits such as lower pollution levels and lower refining costs than, for example, liquid fuels such as gasoline or diesel fuel. Using compressed gas as a vehicle fuel or for industrial applications requires that the gas be transportable, refillable, and safely stored.
The storage of compressed gas, such as hydrogen or compressed natural gas (CNG), is particularly challenging, as the gas must be stored at a very high pressure in order to achieve acceptable storage density. Given the high pressure level needed to store a sufficient amount of gas, leaks can occur at fittings, or gas can permeate through the walls of the pressure vessel used to store the gas.
The amount of permeation of gas through the walls of a pressure vessel is based on the product of a material permeation coefficient, the surface area of the pressure vessel walls in contact with the gas, and the pressure level of the gas divided by the sectional thickness of the pressure vessel wall material. Thus, the resistance to permeation of a pressure vessel constructed of a particular material is proportional to the surface area of the walls and pressure of the gas and inversely proportional to the material thickness of the pressure vessel.
Gas permeation levels are regulated to reduce environmental and user impact. The current NGV2 standard allows a steady state permeation rate of fuel lost to the atmosphere of 0.25 cc of natural gas per hour per liter of water capacity. Given a 40 DGE pressure vessel (550 liters of water capacity), 3.5 cubic feet, or approximately one percent, of the natural gas within the pressure vessel can permeate the walls of the pressure vessel each year. Though this is a relatively small amount, gas permeation near the NGV2 standard levels can still cause a safety concern or an olfactory nuisance in enclosed spaces such as homes or garages, forcing some compressed gas vehicle owners to park vehicles in open spaces. Mercaptans from natural gas are designed to be detected by the human nose at 1,000 ppm, so even small amounts of permeation can be undesirable to, for example, users of CNG pressure vessels.
Compressed gas pressure vessels are currently designed to restrict permeation to meet or exceed regulatory levels using expensive materials or other suboptimal solutions such as increased material thickness to improve mechanical and chemical resistance to permeation. Increasing material thickness adds weight to pressure vessels that are already quite heavy based on design requirements to withstand high pressures from the stored compressed gas. This is undesirable for the vehicle manufacturer.
Another existing solution to address gas permeation is to use an unsealed, vented cabinet to hold a pressure vessel. However, these cabinets can release permeated gas in an uncontrolled manner or the user is required to occasionally vent the compressed gas from the cabinet to the atmosphere to avoid high concentrations in the enclosed cabinet. This type of cabinet cannot safely contain permeated gas for any period of time.